Process for the production of light-colored pastes of α-sulfofatty acid alkyl ester alkali metal salts

ABSTRACT

In the production of light-colored pastes of α-sulfofatty acid alkyl ester alkali metal salts by reaction of fatty acid alkyl esters with gaseous SO 3 , subsequent after-reaction in liquid phase, neutralization with aqueous alkali metal hydroxide solutions and bleaching, effective bleaching can be obtained without any reduction in the washing-active substance content by addition of the hydrogen peroxide during neutralization of the α-sulfofatty acid alkyl ester and subjecting the neutralization product obtained to a temperature-controlled after-reaction.

BACKGROUND OF THE INVENTION

This invention relates to a process for the production of light-coloredpastes of α-sulfofatty acid alkyl ester alkali metal salts, in whichhydrogen peroxide is used as sole bleaching agent to bleach thesulfonation product during neutralization of the α-sulfofatty acid alkylester.

STATEMENT OF RELATED ART

α-Sulfofatty acid alkyl ester alkali metal salts are acquiringincreasing significance as surfactants for detergents and cleaningpreparations based on renewable natural raw materials. In knownprocesses, the α-sulfofatty acid alkyl ester alkali metal salts areobtained in the form of aqueous solutions or pastes by neutralization ofα-sulfofatty acid alkyl esters which may be synthesized by reaction oflower fatty acid alkyl esters with gaseous SO₃. In the final analysis,the basis for the production of the α-sulfofatty acid alkyl ester alkalimetal salts are fats and oils of natural origin from which the lowerfatty acid alkyl esters are obtained by lipolysis and subsequentesterification of the free fatty acids with lower alkanols or bytransesterification of the natural triglycerides with lower alkanols. Inboth reactions, methanol is preferably used as the lower alkanol. Thelower fatty acid alkyl esters are mixtures in which C₆₋₂₂ fatty acidresidues occur, the chain length distribution being dependent on theorigin of the natural fats or oils. In many cases, these fatty acidmixtures are not used for the synthesis as such, but rather in the formof certain fractions. Sulfonation of the fatty acid ester mixtures withgaseous SO₃ gives acidic α-sulfofatty acid alkyl esters which areconverted into aqueous pastes of α-sulfofatty acid alkyl ester alkalimetal salts by neutralization to a pH value of 6 to 8.

The crude α-sulfofatty acid alkyl esters and their alkali metal saltsare more or less colored products which generally have to be treatedwith typical bleaches before and/or after neutralization. Variousbleaching processes have become known in connection with the productionof light-colored α-sulfofatty acid alkyl ester alkali metal saltsolutions and pastes. Combined bleaching processes have been developedfor effective product lightening. According to DE-AS 12 34 709, theacidic α-sulfofatty acid alkyl ester is initially treated with aqueoushydrogen peroxide solution in a first bleaching step. The partlybleached sulfonation product is then neutralized before it is exposed tothe effect of more hydrogen peroxide solution or aqueous hypochloritesolution in a second bleaching step. According to DE-OS 33 19 591, theneutralized sulfonation product is first bleached with aqueoushypochlorite solution at pH values of 7 to 10. Hydrogen peroxidesolution is then added at pH values of ≦7 to stabilize the color valuesreached. These processes are attended by the disadvantage that thedesired high washing active substance contents of the α-sulfofatty acidalkyl ester alkali metal salt pastes, which are reached duringneutralization, are reduced again by introduction of the aqueous bleachsolutions. In the context of the present invention, washing-activesubstance (WAS) is understood to be the sum of α-sulfofatty acid alkylester alkali metal salt and the α-sulfofatty acid ester always presentas secondary product.

One particular difficulty involved in the production and handling ofaqueous α-sulfofatty acid alkyl ester alkali metal salt pastes arisesout of their viscosity behavior in dependence upon the concentration ofwashing-active substance. In aqueous compositions, the α-sulfofatty acidalkyl ester alkali metal salts produced by conventional industrialprocesses (hereinafter also referred to in short as ester sulfonates)only form solutions or suspensions of such low viscosity that they flowsufficiently freely to guarantee uninterrupted completion of industrialprocesses at WAS contents of up to about 40% by weight and then againbeyond solids contents of around 55% by weight. In the intermediateconcentration range, i.e. at WAS contents of around 40 to 55% by weight,the aqueous ester sulfonate compositions show extremely high viscosityvalues, assuming the form of more or less solid gels which can neitherbe stirred nor pumped. In addition, the lower and upper limits of theindividual viscosity maxima can vary by ±5% by weight WAS content. As aresult of this particular concentration/viscosity behavior, estersulfonate pastes having WAS contents above 35 to 40% by weight cannot beobtained simply by neutralization of the acidic α-sulfofatty acid alkylesters with the calculated quantity of aqueous alkali metal hydroxidesolutions using known methods. After the lower limit to the viscositymaximum has been exceeded, the reacting mixture loses its stirrabilityand miscibility. The lack of stirrability and miscibility preventsadequate and rapid dissipation of the heat of neutralization. Localconcentration and temperature peaks initiate unwanted secondaryreactions, more particularly cleavage of the ester bonds present in theester sulfonates, so that undesirably high concentrations of alkalimetal disalts of the free α-sulfofatty acids are established in the endproduct. The subsequent processing of ester sulfonate pastes immobilizedby the high increase in viscosity is of course also impaired to thepoint where it is no longer possible solely as a result of the fact thatthe aqueous compositions in question can no longer be poured or pumped.

The formation of disalts of the free α-sulfofatty acid alkyl esters isundesirable for several reasons. The disalts show only limitedsolubility in water and, in addition, have inadequate surface-activeproperties. Above all, however, disalts as secondary products in estersulfonate pastes have a considerable viscosity-increasing effect.

There has been no shortage of attempts in the past to eliminate at leastmost of the unfavorable effects caused by the particularconcentration/viscosity behavior of the ester sulfonates and theunwanted formation of α-sulfofatty acid disalts. Thus, it has beenproposed to improve the flow behavior of aqueous ester sulfonatecompositions by the addition of flow aids. According to DE-OS 33 05 430,aliphatic alcohols containing 8 to 40 carbon atoms and 1 to 6 hydroxylgroups, alkylphenols and adducts of up to 20 mol ethylene oxide and/orpropylene oxide with the alcohols and alkylphenols mentioned are used asviscosity regulators.

In connection with the unwanted formation of disalts during working upof the acidic α-sulfofatty acid alkyl esters, DE-OS 31 23 681 describesa process in which the neutralizing treatment is carried out in twosteps. In the first step, neutralization is carried out to a pH value of2.5 to 4 with a 15 to 50% by weight alkali metal hydroxide solution inthe presence of a C₁₋₄ alcohol, preferably methanol, in a quantity of 5to 20% by weight, based on the weight of the sulfonated product, beforea final pH value of 6 to 7 is adjusted in the second neutralization stepusing a more heavily diluted alkali metal hydroxide solution. It is saidto be possible by this process to reduce the disalt content of the estersulfonate compositions to 5% by weight, based on washing-activesubstance, or less. A serious disadvantage of this process is obvious:the ester sulfonate pastes produced in this way contain considerablequantities of alcohol which are troublesome in the production ofdetergent mixtures by spray drying insofar as they can cause unwantedpluming. To limit the alcohol content of the end products, DE-OS 33 34517 suggests carrying out the optional bleaching step and neutralizationof the crude α-sulfofatty acid alkyl esters in the presence of such aquantity of a lower alcohol that an aqueous slurry containing 30 to 40%by weight and, based on the weight of the α-sulfofatty acid ester salt,5 to 15% by weight of a lower alcohol sulfate and 8 to 40% by weight ofthe lower alcohol is obtained. Finally, the aqueous slurry is said to beconcentrated to such an extent that it contains 40 to 65% by weightα-sulfofatty acid ester salt, 2 to 10% by weight lower alcohol sulfateand at most 2% by weight lower alcohol.

According to DE-OS 34 32 324, the disalt content of α-sulfofatty acidalkyl ester alkali metal salt pastes can be controlled and reduced bysubjecting the crude sulfonation product before treatment with anaqueous medium to a transesterification reaction in which at least 0.5mol-equivalent alcohol, based on the SO₃ which is not used for theα-sulfonation, is used. According to DE-OS 35 38 910, α-sulfofatty acidalkyl ester salt pastes having solids contents above 35% by weight canbe produced by subjecting the crude ester sulfonates totransesterification in accordance with DE-OS 34 32 324 and thenestablishing solids contents of more than 35% by weight in the aqueouspastes during subsequent working up by neutralization with or withoutpreliminary or subsequent bleaching.

The problem addressed by the present invention was to find a processwhich would enable the necessary bleaching of the sulfonation productswith aqueous bleaching solutions in the production of α-sulfofatty acidalkyl ester alkali metal salt pastes to be carried out in such a waythat the solids content of the pastes would not be reduced by thebleaching. More particularly, the invention set out to provide a processin which free-flowing and pumpable ester sulfonate pastes having WAScontents of 60% to 70% by weight would be obtained by directneutralization of the acidic α-sulfofatty acid alkyl esters with aqueousalkali metal hydroxide solutions and correspondingly adapted bleachingwithout any need for the addition of "foreign" substances, such asrelatively long chain aliphatic mono- and polyalcohols and alkyleneoxide adducts thereof or short-chain alcohols.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the neutralization-bleachingprocess.

DESCRIPTION OF THE INVENTION

The teaching according to the invention is based on the surprisingobservation that light-colored pastes of α-sulfofatty acid alkyl esteralkali metal salts can be obtained without any reduction in the WAScontent of the end product providing hydrogen peroxide is added as solebleaching agent during neutralization of the acidic sulfonation productand the neutralization product is allowed to after-react at elevatedtemperature. It has also been found that light-colored α-sulfofatty acidalkyl ester alkali metal salt pastes having WAS contents of 60 to 70% byweight can be obtained in the absence of viscosity-regulating additivesby introducing the acidic sulfonation product together with aqueousalkali metal hydroxide solution and aqueous hydrogen peroxide solutioninto an already present aqueous phase during neutralization and ensuringthat the pH value of the aqueous phase remains within a certain range.

The present invention relates to a process for the production oflight-colored pastes of α-sulfofatty acid alkyl ester alkali metal saltsby reaction of fatty acid alkyl esters with gaseous SO₃, subsequentafter-reaction in liquid phase, neutralization with aqueous alkali metalhydroxide solutions and treatment with hydrogen peroxide, in which thehydrogen peroxide is added to the α-sulfofatty acid alkyl ester duringneutralization and the neutralization product obtained is subjected to atemperature-controlled after-reaction.

According to the invention, 0.5 to 3% by weight and preferably 1.5 to2.5% by weight hydrogen peroxide, based on washing-active substance inthe neutralization product, is added during neutralization. The hydrogenperoxide is counted as 100% by weight substance. The hydrogen peroxideis added in the form of 5 to 35% by weight and preferably 10 to 20% byweight aqueous solutions. The temperature-controlled after-reaction ofthe neutralization product is preferably carried out at 60° to 90° C.

In one particular embodiment of the invention, at least one typicalbleach activator is added during or after neutralization, but before thetemperature-controlled after-reaction. Suitable bleach activators are,in particular, organic compounds which are capable of formingpercarboxylic acids or percarboxylic acid anions with hydrogen peroxideunder the conditions of the neutralization step and thetemperature-controlled after-reaction. Among these compounds, thosewhich contain N-acyl groups, preferably N-acetyl groups, areparticularly suitable. Special examples of preferred bleach activatorsare tetraacetyl ethylenediamine and tetraacetyl glycoluril. The bleachactivators are added to the reaction mixture in quantities of 0.1 to 5%by weight, based on the washing-active substance expected in theneutralization product.

During the temperature-controlled after-reaction, the neutralizationproduct containing hydrogen peroxide and optionally bleach activator ispreferably kept at the intended temperature of 60° to 90° C. until ithas a Klett color value of ≦200 and preferably ≦100 (as measured on anaqueous solution containing 5% by weight washing-active substance in a 4cm cuvette with a blue filter at 420 nm).

In certain cases, particularly when fairly strongly colored sulfonationproducts are to be processed, it may be advisable to add hydrogenperoxide once more to the neutralization product before the beginning ofthe after-reaction. In that case, generally another 0.1 to 2% by weightand preferably another 0.1 to 0.5% by weight, based on thewashing-active substance present, is added.

In another preferred embodiment of the process according to theinvention relating to the production of highly concentrated α-sulfofattyacid alkyl ester alkali metal salt pastes, particularly those having WAScontents of 60 to 70% by weight, the hydrogen peroxide and the bleachactivator optionally used are introduced together with the sulfonationproduct and the aqueous alkali metal hydroxide solution into an aqueousphase containing 0 to 70% by weight washing active substance at a pHvalue in the range from 2 to 8 and WAS contents of 60 to 70% by weightand preferably 60 to 65% by weight are established during theneutralization of the acidic α-sulfofatty acid ester.

In one particular embodiment of the invention, hydrogen peroxide,sulfonation product, aqueous alkali metal hydroxide solution andoptionally bleach activator are simultaneously introduced into anaqueous phase which initially contains 0% by weight washing-activesubstance, i.e. consists of water.

If solutions having a washing-active substance content which isdifferent from 0 and may be as high as 55% by weight are used as aqueousphase at the beginning of neutralization, these solutions are adjustedbeforehand to a pH value in the range from 2 to 8.

In one preferred embodiment of the invention, a pH value in the rangefrom 2 to 6 and preferably in the range from 3 to 5 is maintained in theaqueous phase during neutralization until a content of washing-activesubstance of 55 to 65% by weight and preferably 60 to 65% by weight isreached. In addition, a pH value in the range from 5 to 8 and preferablyin the range from 5.5 to 7.5 is preferably adjusted and maintained inthe aqueous phase after a washing-active substance content of 55 to 65%by weight and preferably 60 to 65% by weight has been reached.

In another preferred embodiment of the invention, hydrogen peroxide,sulfonation product, aqueous alkali metal hydroxide solution andoptionally bleach activator are simultaneously introduced into anaqueous phase initially containing at least 55% by weight washing-activesubstance. In this case, a pH value of 5.5 to 7.5 is preferablymaintained in the aqueous phase during neutralization.

The neutralization of the α-sulfofatty acid alkyl ester is carried outat temperatures below 95° C. and preferably at temperatures in the rangefrom 60° to 80° C.

Neutralization of the acidic α-sulfofatty acid alkyl ester is bestcarried out in a neutralization loop of the type diagrammaticallyillustrated in FIG. 1. The predominant part of the aqueous phase isaccommodated in the stirred tank 1 in which it is continuously stirredby the stirrer 2. Aqueous phase is continuously removed by thecirculation pump 4 via the circulation pipe 3 and is cooled to thenecessary extent in the cooler 5 provided to control the reactiontemperature. The α-sulfofatty acid alkyl ester to be neutralized isintroduced into the stream of the circulated aqueous phase through thepipe 6. Aqueous alkali metal hydroxide solution having a standardconcentration, for example 50% by weight sodium hydroxide solution, isintroduced into the circuit through the pipe 7. The concentration of thestandard alkali metal hydroxide solution can be reduced to theparticular value required before it is introduced into the productcircuit by the introduction of water through the pipe 8. Aqueoushydrogen peroxide solution can be introduced into the neutralizationcircuit through the pipe 9. Bleach activators optionally added may beintroduced into the circulating aqueous phase through the pipe 10. Themixture of acidic α-sulfofatty acid alkyl ester, alkali metal hydroxidesolution and circulated aqueous phase then enters the mixer 11 forfurther homogenization and, from the mixer 11, is transported into thestirred tank 1 through the last section of the circulation pipe 3. Theα-sulfofatty acid alkyl ester alkali metal salt paste formed duringneutralization can be removed through the pipe 12. A neutralization loopof the type described in the foregoing may be made up exclusively ofstandard units, fittings and pipes. Known measuring and control methodsfor chemical processes may be used for the necessary monitoring of thepH value and the reaction temperature and for controlling the productand coolant flows.

The after-reaction which follows neutralization may be carried out in aheatable stainless steel vessel of known type provided with a stirrer.After leaving the neutralization loop, the neutralization product isbest defoamed in a vessel kept under reduced pressure, for example 300to 400 mbar, before it is introduced into the after-reactor.

Using the described neutralization loop, the neutralization process mayreadily be set up for continuous operation. In this case, the aqueousphase pump-circulated in the neutralization loop consists of anα-sulfofatty acid alkyl ester alkali metal salt paste which correspondsto the neutralization product removed from the circuit in its WAScontent, its hydrogen peroxide content and its bleach activator content,if any. Acidic α-sulfofatty acid alkyl ester, alkali metal hydroxidesolution, hydrogen peroxide solution and, optionally, bleach activatorare added to the neutralization circuit at the same rate asneutralization product is removed from the circuit. To maintain aconstant WAS content, the standard alkali metal hydroxide solutionintroduced is diluted to the necessary concentration by addition ofwater.

In the context of the invention, fatty acid alkyl esters are understoodto be lower alkyl esters of saturated fatty acids, more particularlyesters of fatty acids containing 10 to 18 carbon atoms and saturatedaliphatic alcohols containing 1 to 4 carbon atoms. Basically, individualfatty acid alkyl esters may be used as starting material. In general,however, ester mixtures of the type obtainable from fats and oils ofnatural origin either by ester cleavage and subsequent esterificationwith lower alkanols or by transesterification with lower alkanols byknown methods are used as the starting material, the corresponding fattyacid methyl ester mixtures being preferred. If the fatty acid estermixtures obtained in this way have relatively large percentage contentsof esters of fatty acids containing less than 10 carbon atoms, these"head-fractionated fatty acid esters" are generally removed bydistillation. Apart from the CH₂ group in the α-position to the estergroup, the fatty acid esters should not contain any sulfatable orsulfonatable groups. For this reason, hydroxyfatty acid esters ormixtures containing hydroxyfatty acid esters are not suitable asstarting materials. Fatty acid ester mixtures containing non-negligiblequantities of esters of unsaturated fatty acids, more particularlyesters having an iodine value above 5, are only suitable as startingmaterials after saturation of the double bonds in the course ofhardening by hydrogenation using known methods. During thehydrogenation, the iodine values of the ester mixtures are preferablyreduced to values of 0.2 and lower.

The fatty acid esters are sulfonated with gaseous SO₃ as the sulfonatingagent at temperatures in the range from 30° to 80° C. The SO₃ iscontacted with the fatty acid esters after dilution with air ornitrogen, preferably in the form of a gas mixture containing 1 to 10% byvolume SO₃. The quantity of SO₃ is gauged in such a way that the molarratio of fatty acid ester to SO₃ is in the range from 1:1.2 to 1:1.8.This reaction may be carried out in standard reactors suitable for thesulfonation of organic compounds, such as fatty alcohols, alkyl benzenesor olefins, more particularly in falling-film reactors or multistagecascades of stirred tank reactors.

The crude sulfonation product issuing from the sulfonation reactor stilldoes not have the desired degree of sulfonation. For this reason, thecrude reaction product is delivered immediately after sulfonation to asuitable apparatus in which it is subjected to a temperature-controlledafter-reaction for 20 to 40 minutes and preferably for 25 to 35 minuteswith mechanical agitation until the desired degree of sulfonation isreached. The apparatus required for this reaction step may consist of astandard reactor with a heating and cooling circuit, a standardtemperature-controlled pipe coil or a standard cascade of stirred tanks.The after-reaction is carried out at temperatures of 60° to 100° C. Thesulfonated product may be mechanically agitated during theafter-reaction by stirring, by introduction of the product underpressure, by the installation of chicane-like baffles in the apparatusor, where a pipe coil is used, by generation of turbulent flow. Theafter-reaction of the sulfonated product may be controlled by suitablechoice of the parameters mentioned, more particularly the reaction time,in such a way that a degree of sulfonation of at least 90% andpreferably from 94 to 98% is reached.

Following the after-reaction, the aged sulfonation product is subjectedto neutralization and bleaching in accordance with the invention.

EXAMPLE

The starting material used was a technical palmitic/stearic acid methylester (in % by weight according to chain length in the fatty acid part:0.2 C₁₂ ; 1.2 C₁₄ ; 61.4 C₁₆ ; 0.9 C₁₇ ; 35.9 C₁₈ ; 0.4 C₂₀ ; averagemolecular weight 281.5; acid value 1.1; iodine value 0.1; saponificationvalue 202.1). The fatty acid methyl ester was continuously sulfonatedwith an SO₃ /air mixture (5% by volume SO₃) in a molar ratio of 1:1.25in a standard falling-film reactor at a temperature of 80° C. Theresulting reaction mixture was subjected to an after reaction in aholding-time cascade of four stirred tanks with a holding time of 25minutes. Thereafter the acid value of the sulfonation product was 198.The degree of sulfonation was 96%.

616 kg water were introduced into and pump-circulated in aneutralization loop of the described type. 6836 kg of the agedsulfonation product described above and 1890 kg 50% by weight sodiumhydroxide solution were initially fed into the circuit of the aqueousphase at such a rate that a pH value of 5 was maintained in the aqueousphase. When the WAS content of the aqueous phase had reached 55% byweight, the inflow rates of the sulfonation product and the sodiumhydroxide solution were adjusted in such a way that the pH value in theaqueous phase was 6. During introduction of the sulfonation product andthe aqueous sodium hydroxide solution, 658 kg 20% by weight aqueoushydrogen peroxide solution (2% by weight, based on expectedwashing-active substance) were introduced into the neutralizationcircuit. After the entire acidic sulfonation product had been introducedinto the neutralization circuit, the pH value of the aqueous phase wasincreased to 6.1 by addition of the remaining sodium hydroxide solution.Throughout the neutralization process, the reaction temperature was keptat 90° to 93° C. The aqueous phase could readily be stirred andpump-circulated at any time during the neutralization process.

The neutralized product was pumped off into a closed vessel and defoamedunder reduced pressure (approx. 380 mbar) before it was transferred to astirrer-equipped stainless steel vessel and exposed therein to atemperature of 75° C. for 38 hours while mixing. The degree oflightening as a function of time was monitored by measurement of theKlett color values of samples. The Klett color values were measured onaqueous solutions containing 5% by weight washing-active substance in a4 cm cuvette using a blue filter (420 nm). The bleaching trend as afunction of time is shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Lightening during the bleaching reaction at 75° C.                     Reaction time (n)                                                                            Klett color value                                              ______________________________________                                         0             640                                                            12             232                                                            16             150                                                            20             130                                                            38              80                                                            ______________________________________                                    

10,000 kg of a stirrable and pumpable α-sulfofatty acid methyl estersodium salt paste having a washing-active substance content of 66.0% byweight (54% by weight α-sulfofatty acid methyl ester sodium salt and 12%by weight α-sulfofatty acid disodium salt) were obtained.

What is claimed is:
 1. A process for the production of a light coloredpaste of an α-Sulfofatty acid alkyl ester alkali metal salt whichcomprises: (1) contacting a fatty acid alkyl ester with gaseous sulfurtrioxide to form a partially sulfonated product; (2) maintaining saidpartially sulfonated product at a temperature of from about 60° C. toabout 100° C. for a time to form a sulfonated product having a degree ofsulfonation of at least 90%; (3) bleaching and neutralizing saidsulfonated product with an aqueous solution comprised of an alkali metalhydroxide and hydrogen peroxide; and (4) treating the mixture from step(3) at a temperature of from about 60° C. to about 90° C. to form saidlight colored paste.
 2. The process of claim 1 wherein the concentrationof hydrogen peroxide in said aqueous solution in step (3) is from about0.5% to about 3% by weight of said sulfonated product.
 3. The process ofclaim 1 wherein in step (3) the concentration of said hydrogen peroxidein the aqueous solution is from about 1.5% to about 2.5% by weight ofsaid sulfonated product.
 4. The process of claim 1 wherein said aqueoussolution in step (3) is further comprised of a bleach activator.
 5. Theprocess of claim 4 wherein said bleach activator is a compound whichforms a percarboxylic acid or a percarboxylic acid anion as a result ofreaction with hydrogen peroxide.
 6. The process of claim 5 wherein saidbleach activator is an organic compound having an N-acyl group.
 7. Theprocess of claim 6 wherein said acyl group is an acetyl group.
 8. Theprocess of claim 5 wherein the concentration of said bleach activator isfrom about 0.1% to about 5% by weight of said sulfonated product in step(3).
 9. The process of claim 1 wherein step (3) is carried out by addingsaid sulfonated product, hydrogen peroxide, and alkali metal hydroxideto an aqueous phase comprising from 0 to about 55% by weightwashing-active substance and having a pH of from about 2 to about 8 toform in step (4) a light colored α-sulfofatty acid alkyl ester alkalimetal salt paste comprised of from about 60% to about 70% by weight ofwashing-active substance.
 10. The process of claim 9 wherein said pH isfrom about 5 to about 8 after a total of washing-active substance offrom about 55% to about 65% by weight is achieved.
 11. The process ofclaim 9 wherein said pH is from about 5.5 to about 7.5 after a total ofwashing-active substance of from about 60% to about 65% by weight isachieved.
 12. The process of claim 1 wherein step (3) is carried outbelow about 95° C.
 13. The process of claim 12 wherein step (3) iscarried out at a temperature of from about 60° C. to about 90° C. 14.The process of claim 1 wherein said fatty acid alkyl ester is an esterof a fatty acid having from about 10 to about 18 carbon atoms and asaturated aliphatic alcohol having from 1 to about 4 carbon atoms. 15.The process of claim 1 wherein said fatty acid alkyl ester is a methylester.
 16. The process of claim 1 wherein said fatty acid alkyl ester isa mixture of fatty acid methyl esters obtained by transesterification ofnatural fats or oils with methanol.
 17. The process of claim 1 whereinsulfur trioxide is part of a mixture comprised of up to 10% by volumesulfur trioxide and the remainder air or up to 10% by volume sulfurtrioxide and the remainder nitrogen.
 18. The process of claim 1 whereinthe molar ratio of sulfur trioxide to fatty acid alkyl ester is fromabout 1.1/1.0 to about 1.8/1.0.
 19. The process of claim 18 wherein themolar ratio of sulfur trioxide to fatty acid alkyl ester is about1.1/1.0.
 20. The process of claim 1 wherein the degree of sulfonation instep (2) is from about 94% to about 98%.